1. Field of the Invention
This invention relates generally to compositions and devices for harvesting electrical energy from mechanical and thermal energy, storing such produced energy and/or sensing strain and methods regarding same.
2. Background of the Invention
Energy harvesting from ambient environment has become an exciting research field [Sodano, H. A., et al., Comparison of Piezoelectric Energy Harvesting Devices for Recharging Batteries, Journal of Intelligent Material Systems and Structures 16, 799-807 (2005)] due to their possible applications in a range of areas including smart textiles, self-powered sensors, and electronics. Developing new and facile approaches to build multifunctional materials to be used in such smart applications is of great interest. [Sodano, H. A., et al., Estimation of Electric Charge Output for Piezoelectric Energy Harvesting, Strain, 40, 49-58 (2004)]. Recent reports have focused on using vapor phase grown nanowires of piezoelectric materials on solid substrates to build such energy harvesting systems. [Wang, Z. L., et al., Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays, Science, 312, 242 (2006) (“Wang 2006”)].
The growing need for renewable alternative sources of energy has motivated significant effort to develop new forms of energy conversion and storage devices. [Leijon, M., et al., Economical considerations of renewable electric energy production—especially development of wave energy, Renewable Energy, 28, 1201(2003) (“Leijon 2003”)]. Conversion of mechanical energy to electrical energy could play a key role in developing remote access electronic devices, self-powered sensors or implantable medical devices. This can be done using piezoelectric materials and such energy harvesting idea has been shown to be attractive because of their capability for harvesting energy from unused power in various forms (e.g. vibrations, flowing water, wind, human motion and shock waves). [Beeby, S. P., et al., Energy harvesting vibration sources for Microsystems applications, Meas. Sci. Technol., 17, R175 (2006)]. Several types of piezoelectric materials such as ceramics (lead zirconium titanate), polymers (poly(vinylidene fluoride)) and macro fiber composites have been successfully used for harvesting energy under a large spectrum of vibration frequencies. [Polla, D. L., et al., Processing and Characterization of Piezoelectric Materials and Integration into Microelectromechanical Systems, Annu. Rev. Mater. Sci., 28, 563(1998); Zhou, J., et al., Dissolving Behavior and Stability of ZnOWires in Biofluids A Study on Biodegradability and Biocompatibility of ZnO Nanostructures, Adv. Mater., 18, 2432 (2006) (“Zhou 2006”)]. Among these, ZnO is a unique material that combines semiconductor and piezoelectric properties [Zhou 2006] and the ease of low cost manufacturing makes it ideal candidate for energy harvesting applications. [Zhou 2006; Wang, X., et al., Direct Current Nanogenerator Driven by Ultrasonic Waves, Science, 316, 102 (2007) (“Wang I 2007”); Wang, X., et al., Integrated Nanogenerators in Biofluid, Nano Lett. 7, 2475 (2007) (“Wang II 2007”); Wang, Z. L., Nanopiezotronics, Adv. Mater., 19, 889 (2007) (“Wang III 2007”); Gao, P. X., et al., Nanowire Piezoelectric Nanogenerators on Plastic Substrates as Flexible Power Sources for Nanodevices, Adv. Mater., 19, 67 (2007) (“Gao 2007”); Liu, J., et al., Carrier Density and Schottky Barrier on the Performance of DC Nanogenerator, Nano Lett., 8, 328 (2008) (“Liu I 2008”); Qin, Y., et al., Microfibre-nanowire hybrid structure for energy scavenging, Nature, 451, 809 (2008) (“Qin 2008”); Liu, J., et al., Toward high output-power nanogenerator, Appl. Phys. Lett., 92, 173105(2008) (“Liu II 2008”); Lao, C. S., et al., ZnO Nanobelt/Nanowire Schottky Diodes Formed by Dielectrophoresis Alignment across Au Electrodes, Nano Lett., 6, 263 (2006)].
The piezoelectric properties of both bulk and nanostructured zinc oxide have been studied both experimentally and theoretically. [See, e.g., Schubert, M. A., et al., Finite Element method calculations of ZnO nanowires for nanogenerators, Appl. Phys. Lett., 92, 122904 (2008); Lovesey, S. W., et al., Polar multipoles in wurtzite-like crystals (ZnO, GaN), J. Phys.: Condens. Matter, 20, 122201(2008); Wu, X., et al., Systematic treatment of displacements, strains, and electric fields in density functional perturbation theory, Phys. Rev. B, 72, 0335105 (2005); Hill, N. A., et al., First-principles study of strain-electronic interplay in ZnO: Stress and temperature dependence of the piezoelectric constants, Phys. Rev., B 62, 8802 (2000); Xiang, H. J., et al., Piezoelectricity in ZnO nanowires: A first-principles study, Appl. Phys. Lett., 89, 223111(2006)]. Wang and coworkers have developed aligned arrays of zinc oxide nanowires by vapor-liquid-solid process on GaN and sapphire substrates and have utilized them for current generation based on the deflection/vibration of the nanowires. [Zhou 2006; Wang I 2007; Wang II 2007; Wang III 2007; Gao 2007; Liu I 2008; Qui 2008]. In all these, fabrication procedures involve multi step materials processing methods and difficult fabrication of devices using precise manipulators, making it challenging for scalable and cost-effective manufacturing of devices. Accordingly, there is a need for innovative, inexpensive, scalable technologies based on new materials and engineering approaches.
Critical infrastructure including highways, buildings, bridges, aircrafts, ships, pipelines, etc., form the lifeline of economic and industrial hubs and are sometimes subjected to severe loading conditions due to extreme events such as earthquakes, hurricanes and other natural disasters during their lifetime. In order to prevent catastrophic failures and subsequent loss of life it is essential to continuously monitor the state of the structure and identify any initiation of damage in real time. Structural health monitoring (SHM) provides an autonomous way of tracking changes in the system in real time using a combination of instrumentation systems and analytical methods. Instrumentation systems consist primarily of transducers to measure physical quantities such as displacements, accelerations etc. which can give insight into the behavior of structures. Among the quantities of interest for SHM, strain is a local and direct measure of the state of the structure and hence is widely used as a reliable indicator of the damage induced in the structure. Strain sensors hence are extensively used in SHM applications. [Ausanio, G., et al., Magnetoelastic sensor application in civil buildings monitoring, Sensors Actuators: A Physical, 123/124, 290 (2005); Ansari F, Fiber optic health monitoring of civil structures using long gage and acoustic sensors, Smart Mater. Struct., 14, Si (2005)].
Strain gauges or transducers can be broadly classified into optical sensors, resistance based sensors and piezoelectric sensors. Among them, resistance based sensors form the major portion of commercially available foil strain gage sensors. Recent research in development of resistance sensors based on carbon nanotubes [Dharap, P., et al., Nanotube film based on SWNT for macrostrain sensing, Nanotechnology Journal, 15(3), 379 (2004)] and their composites [Kang, I., et al., A carbon nanotube strain sensor for structural health monitoring, Smart Mater. Struct. 15, 737 (2006) (“Kang 12006”); Ramaratnam A, et al., Reinforcement of piezoelectric polymers with carbon nanotubes: pathway to next generator sensors, Journal of Intelligent Materials Systems and Structures, 17, 199 (2006)] has accelerated since the discovery of carbon canotubes (CNTs) and their excellent electro-mechanical properties. However, CNT based sensor technology is yet to find commercial applications and is still in the development stage. Further power requirements are a major limitation and a bottle neck for large scale deployment of instrumentation for SHM. Hence, strain transducers with low power requirements are highly desirable.
Piezoelectric materials have the ability to convert mechanical energy into electrical energy and have long been used for strain sensing. Among other type of sensors, piezoelectric sensors have lowest power requirements [Park, G., et al., Energy Harvesting for Structural Health Monitoring Sensor Networks, Journal of Infrastructure Systems, Vol. 14, No. 1, 64 (2008)] and the charge output from piezoelectric sensors lie within the range of measurement capabilities of commercially available A/D sensors. Lead Zirconate Titanate (PZT) is a common piezoelectric material that is commercially used for piezoelectric actuators and sensors. Recently, Wong and co-workers fabricated ZnO piezoelectric fine wire based strain sensor, wherein ZnO fine wire were laterally bonded with polystyrene (PS) substrate. [Zhou, J., et al., Flexible Piezotronic Strain Sensor, Nano Lett., 8(9), 3035 (2008) (“Zhou 2008”)]. The change in I-V behavior of wire as a function of strain was explained based on combined effects from strain induced band structure change and piezoelectricity result in the change of Schottky barrier height. [Zhou 2008].
Single crystal of piezoelectric materials like PZT or ZnO, used as strain sensor are usually bonded on the surface or embedded inside the host structure for strain measurement and have limitations to measure strain at discrete points and in a fixed direction. [Fixter, L., et al., State of the Art Review—Structural Health Monitoring, Smart Materials, Surfaces and Structures Network (Smart. Mat) Report (2006) (“Fixter 2006”)]. Piezoelectric materials are very hard ceramic materials that are not amenable to flexible uses and very weak in tension. [Fixter 2006]. On the other hand, fabrication of flexible piezoelectric sensors or actuators requires special fabrication techniques making it uneconomical for use on a large scale for SHM applications. Therefore, there is a need to develop simple, inexpensive and flexible sensors that can be embedded into the material or form fitted easily onto an existing structure and used for multidirectional sensing over a practically viable sensing region. Avoiding lead based products is also eco-friendly and essential for use in next generation technology.